The battle with Fire Bird is not a difficult one. Dart's Dragoon magic is virtually useless, but Rose's can be helpful. Having either Rose or Shana throw a Spear Frost or other Water-based Attack Items can deal a heavy amount of damage to him. Other than that, simply attack him as much you can. Fire Bird's attacks can be painful, especially the magical ones when attacking Lavitz. He can also summon four "Volcano Balls" to attack your party.